1. Field of the Invention
The invention relates to a sigma-delta modulator for converting an input signal into a 1-bit digital output signal, comprising:
a low-pass filter constituted by a series connection of first, second and third sections having at least first order filter functions, each section having an input and having an output for producing respective first, second and third section signals;
quantization means for converting the third section signal into the 1-bit digital output signal and for producing first and second feedback signals in response to the digital output signal;
a first subtractor stage for producing a first difference signal from the second section signal and the first feedback signal and for applying the first difference signal to the input of the third filter section;
a second subtractor stage for producing a second difference signal from the input signal and the second feedback signal and for applying the second difference signal to the input of the first filter section;
coupling means for coupling the first section signal to the input of the second filter section.
2. Description of the Related Art
Such a sigma-delta modulator (SDM) is known from "A 16-bit Oversampling A-to-D Conversion Technology Using Triple-Integration Noise Shaping", IEEE Journal of Solid-State Circuits, Vol. SC-22, No. 6, December 1987, pp. 921-928. Sigma-delta modulation is a technique in which an analog signal is made into a digital signal having high resolution and low quantization noise with the aid of oversampling by quantization means having low resolution and high quantization noise. The digital signal is reconverted to an analog feedback signal by means of a digital-to-analog converter having the same low resolution and is subtracted in a subtracting stage from the analog input signal. The difference between the two signals is filtered in an analog loop filter and applied to the quantization means. The use of a sufficiently high loop gain for baseband frequencies of the analog signal produces a digital signal in which the quantization noise within the baseband is low at the expense of a higher quantization noise above this baseband. By means of digital filter techniques, however, noise above the baseband can be suppressed effectively, for example, by means of a decimating filter that converts the oversampled 1-bit digital signal into a multi-bit digital signal at the desired lower sampling rate.
Sigma-delta modulation to a 1-bit digital signal is advantageous in the simplicity of the quantization means. A simple decision circuit having an inherently good linearity will then be sufficient, while the associated digital-to-analog converter may be omitted. The attainable signal-to-noise ratio in the 1-bit digital signal is determined, for example, by the extent of oversampling and the order of the loop filter. A higher order loop filter is advantageous because it allows a lower oversampling rate while a specific signal-to-noise ratio in the baseband is maintained. The order of the loop filter is limited in practice by stability problems. In the prior art SDM in the aforementioned article, third-order loop filters are used comprising a cascade connection of three first-order high-gain active integrators. With these third-order filters satisfactory results can be achieved when digitizing audio signals in which the signal-to-noise ratio is so high that a 16-bit resolution in the ultimate digital audio signal is attainable. This requires oversampling rates of several MHz.
For digitizing the video signals in the above manner, the prior art 1-bit modulators fail due to the much higher oversampling rate needed. For a digital video signal having 10-bit resolution, a 32-fold oversampling rate of 432 MHz will be necessary if a third order loop filter is utilized. At such high sampling rates, however, the customary implementations of third order loop filters comprising the so-called switched capacitor filters and also RC-active filters will fail, because such filters can no longer process these rates as a result of parasitic influences. The loop gain at the high oversampling rate (clock signal frequency) becomes unpredictable so that nothing will come of the intended third order filter characteristic.